The present invention relates to a radiation detector and in particular for X-ray tomography.
More specifically, the present invention applies to multicellular detectors containing a high pressure gas and which are to be used in tomographic systems which have for example a calculator or computer. In an X-ray tomography apparatus having a computer, the spatial distribution of the intensities of the X-rays which have passed through the object to be visualised in accordance with the sectional plane and in different paths is converted into an electrical signal which is processed so as to make it possible to obtain an image of the object. The detectors used must detect the electromagnetic energy with an adequate spatial resolution. The sampling rate of the signals supplied by the detectors is generally limited by the relaxation time of these detectors. It is therefore necessary to use detectors having low relaxation times, a high sensitivity and a very good spatial resolution. It is known that a multicellular detector having a plurality of spatially separate cells makes it possible to provide an economic and efficient tomography apparatus. In this type of detector, the X-radiation is detected in a gas of high atomic mass under high pressure. The X-photons interact with the gas to produce ion-electron pairs. The electrons are collected on electrodes under the action of an electrical field applied between the electrodes and supply electric currents proportional to the intensity of the X-rays. These electrons are collected on positive electrodes or plates alternating with negative electrodes or plates on which the ions are collected. The electrons and the positive ions which are produced by the interaction of the X-photons and the gas move along lines of force of the electric field and are collected on positive and negative electrodes. All the charges produced by the interaction of the X-rays and the gas must be collected in an as short a time as possible and in particular in a time which is as uniform as possible in the volume of the detection cell, in such a way that the signal corresponding to the following sampling interval is not disturbed.
In general, the positive and negative electrodes are separated by insulants which, unfortunately have a very high sensitivity to X-radiation. This leads to significant stray conduction phenomena. This disadvantage is aggravated by the fact that these phenomena are not quantitatively repetetive and have a significant persistence after irradiation. Moreover, the necessity of increasing the spatial resolution to improve the quality of the image obtained makes it necessary to construct detectors having a very large number of cells. The oldest tomography apparatuses had about 200 detection cells at an angle of 40.degree.. In order to increase the image quality, it is at present necessary to construct apparatuses having 500 or even 1000 cells on the same angle. With a view to significantly decreasing the leakage currents in the insulant separating the electrodes of the detectors it is known that it is possible to use mineral insulants and in particular ceramic insulants. These insulants can be easily worked, are excellent electrical insulators and are dimensionally very stable. However, despite all these qualities, they do not permit the elimination of all the problems caused by leakage currents. The possible improvement of this type of detector consists of eliminating the insulant within the electrode and transferring it to the upper and lower extremities of the electrodes. However, this solution has the disadvantage of making the electrodes less rigid and more fragile, which may lead to the effects of microphony due to a lack of rigidity of the electrodes.